Priority is claimed on Japanese Patent Application No. 2007-045294, filed Feb. 26, 2007, the contents of which at incorporated herein by reference.
1. Field of the Invention
The present invention relates to an impulse line-clogging detecting apparatus and an impulse line-clogging detecting method for detecting clogging of impulse lines in a differential pressure transmitter for measuring a differential pressure of fluid flowing through a piping.
2. Description of Related Art
As is generally known in the related art, a differential pressure transmitter measures a differential pressure between a front stage (high pressure side) and a rear stage (low pressure side) of an orifice provided in a pipe in which fluid flows, based on a pressure transmitted through impulse lines arranged at the front and rear stages of the orifice. If the impulse lines are clogged in such a differential pressure transmitter, it is difficult to measure the differential pressure precisely. Accordingly, it is very important to detect clogging of the impulse lines from a standpoint of fluid management (for example, see Japanese Unexamined Patent Application, First Publication No. 2004-132817). Hereinafter, an example of a conventional method of detecting clogging of an impulse line will be described.
In a normal state (i.e., a state where an impulse line is not clogged), assuming that differential pressure data obtained in time-series from a differential pressure transmitter is Dps(i), differential pressure fluctuation Fdps(i) is expressed by the following equation (1). In the equation (1), Dps(i) is a current value of differential pressure data and Dps(i−1) is a previous value of differential pressure data. A variance (square mean value) Vas of differential pressure fluctuation Fdps(i) is expressed by the following equation (2). In the equation (2), N is the total number of samples of differential pressure data Dps(i). Hereinafter, Vas is referred to as a standard fluctuation variance. The standard fluctuation variance Vas is beforehand obtained in the first step of at actual operation in the normal state.Fdps(i)=Dps(i)−Dps(i−1)  (1)Vas=Σ{Fdps(i)2}/N  (2)Fdps(i)=Dps(i)−2Dps(i−1)+Dps(i−2)  (3)
Next, in an at operation of a plant whenever diagnosis time for impulse lines comes, by using differential pressure data Dp(i) obtained in time-series from a differential pressure transmitter, a differential pressure fluctuation Fdp(i) and a differential pressure fluctuation variance Va are obtained in the same way as the above. The differential pressure fluctuation Fdp(i) and the differential pressure fluctuation variance Va in the actual operation are calculated based on the above equations (1) and (2).
Then, a ratio D (=√(Va/Vas)) of the differential pressure fluctuation variance Va obtained in the diagnosis time for the impulse lines to the beforehand obtained standard fluctuation variance Vas is calculated. Since the differential pressure fluctuation variance Va is varied depending on conditions of clogging of the impulse lines, the clogging of the impulse lines can be detected based on the variation of the ratio D. For example, if both of high and low pressure side impulse lines are clogged the differential pressure fluctuation variance Va becomes small (the ratio D also becomes smaller). If one of the high and low pressure side impulse lines is clogged, the differential pressure fluctuation variance Va comes large (the ratio D also becomes larger). In addition, if the impulse lines are in a normal state, the differential pressure fluctuation variance Va approaches the standard fluctuation variance Vas and the ratio D approaches “1”. Accordingly, by comparing the ratio D with a predetermine threshold value, it can be determined whether one or both of the high and low pressure impulse lines are clogged, or the high and low pressure impulse lines am in the normal state.
The differential pressure fluctuation may be obtained using the equation (3) instead of the equation (1). For example, when the differential pressure fluctuation is obtained using the equation (1), if transitional change (rising or falling) occurs in a differential pressure, the transitional change component may appear as fluctuation. To overcome this problem, the equation (3) is used to remove the transitional change component even if the transitional change occurs in the differential pressure thereby making it possible to detect only a real fluctuation component (for example, see Japanese Unexamined Patent Application, First Publication No. 2004-294175).
In the mean time, the differential pressure fluctuation variance is varied depending on a flow rate as well as clogging of an impulse line. Accordingly, since the standard fluctuation variance Vas is an experimental value under the condition of fixed flow rate, if the flow rate is varied in an actual operation, the ratio D is also varied even in the same clogging of the impulse line. That is, since it is require to change a threshold value or a setting value of the standard fluctuation variance Vas depending on the flow rate, a work load of an operator increases, and apparatus costs increase when the change of the setting value is automated.
The present invention was made in view of the above-mentioned circumstances and has an object of detecting clogging of impulse lines precisely without being influenced by a flow rate of a fluid.